The present invention relates to the field of radar signal processing and, more particularly, to the detection of non-oscillatory motion of objects by measuring the differential sideband power of energy, either acoustical or electromagnetic, received by radar systems.
Energy received by radar receivers contains unwanted signals called "clutter" or "clutter noise" from objects that appear to oscillate either because of their movement or movement of the radar receiver. Signal processing systems connected to radar receivers are designed to reduce such clutter. The simplest method of radar signal processing involves double sideband mixing which uses coherent baseband mixing and low pass filtering to reject components of clutter. This type of radar signal processing, however, yields an ambiguous result because it does not indicate whether the target being tracked is moving toward or away from the radar.
To avoid an ambiguous output, many radar systems instead use single sideband mixing in which two quadrature channels from the radar are mixed to produce signals corresponding to either positive or negative Doppler frequencies. The polarity of the Doppler frequency indicates whether the target being tracked is approaching or retreating from the radar.
A single sideband mixing system, however, is not as effective at eliminating clutter noise as is differential sideband mixing. Differential mixing takes advantage of the fact that the frequency of the Doppler energy from a continuously moving target is either above or below the transmitter frequency. Clutter energy, on the other hand, is approximately equally distributed on both sides of the transmitted signal's frequency.
One differential sideband radar system, is described in U.S. Pat. No. 3,432,855 to Kalmus and is represented in FIG. 1. The radar in that system produces two baseband quadrature signals, I(t) and Q(t) with Q(t) shifted 90.degree. from I(t) by phase-shift circuit (A). I(t) and Q(t) are bandpass filtered by filters F.sub.1 and F.sub.2 to become I'(t) and Q'(t), respectively. The Q'(t) signal is then phase-shifted 90.degree. a second time by phase-shift circuit (B) to yield Q(t) which is multiplied by I'(t). The average of that product is a differential sideband power measurement.
One disadvantage of this system is the difficulty in fabricating the second 90.degree. phase-shift circuit (B) in FIG. 1. Unlike phase-shift circuit (A), which is also a 90.degree. phase-shift circuit, phase-shift circuit (B) must shift signals whose bandwidth is large compared to the center frequency of circuit (B).
Another disadvantage of the system in Kalmus is that additional filters similar to F1 and F2 are required to measure the power in different frequency windows. This makes the entire system complex.
It is an object of the present invention to simplify the radar signal processors used to measure differential sideband power in order to detect non-oscillatory movement of objects by eliminating some of the complex circuitry in such systems and by reducing the number of elements needed to provide accurate radar signal processing.
It is also an object of the present invention to simplify the design of radar signal processors by taking advantage of certain relationships between the frequency windows in which the differential power is being measured.
Additional objects and advantages of the present invention will be set forth in part in the description which follows and in part will be obvious from that description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the methods and apparatus particularly pointed out in the appended claims.